1. Field of the Invention
The present invention relates to a thin film circuit substrate and a process for the manufacture thereof.
2. Description of the Related Art
In recent years, the demand for an acceleration of the processing speed of electronic devices, including computers, has led to a requirement for an acceleration of a signal propagation velocity of a microstrip line on a circuit substrate.
As is widely known, the signal propagation velocity of a circuit line is decided by the dielectric constant of an insulator thereof, and thus various circuit substrates using an insulator with a low dielectric constant have been proposed.
As an insulating material used for a thin film circuit substrate for accelerating the signal propagation velocity thereof, there have been proposed fluorinated polyimides (dielectric constant=2.7), fluoropolymers such as PTFE (dielectric constant=2:1) or amorphous fluoropolymers such as Teflon AF (dielectric constant=2.0 or less), having a dielectric constant lower than those of the usual, widely-used polyimides (dielectric constant=3.3).
The above-described low dielectric constant materials, however, have a defect in that the adhesion thereof to a substrate is low, and further, these materials cannot be applied to a substrate in a large thickness. Furthermore, since Teflon AF materials are damaged by a fluorine organic solvent, these materials have a drawback in that serious arise when they are washed after having been soldered, or when a VPS (vapor phase soldering) is applied thereto.
To solve the above-described problem, a method as illustrated in FIG. 26 may be considered, in which a signal transmission path 1 is sandwiched between an insulator layer 3 made of a normal polyimide with a high adhesion and an insulating layer made of an insulator with a low dielectric constant, whereby an insulating layer is formed. Even by a constitution such as described above, however, a reduction of the dielectric constant can be obtained only to a degree such that the dielectric constant of the finally obtained insulator is reduced at most to about the average value of the dielectric constants of the insulator layer 3 and the insulator 2, and further, such a constitution has a defect of problems with the adhesion strength thereof. In FIG. 26, reference numeral 4 indicates a ground layer.